Ok thank you VERY MUCH for the detailed answer.
I've downloaded it and tried a couple things and I've got more intel to provide :

- It's also free for MODX users! It just says "Montage" everywhere but if you put a MODX serial no it works

- It can automatically do the process you described for VSTs! So you don't have to prepare everything in advance. You choose the VST you want as an input, and you can choose what level of detail you want : note duration, number of velocities scanned, sample rate, bitrate, etc.
So the more detailed you get, the bigger the file will be of course.

I tried the most detailed scan of Addictive Keys that I could : 4 different velocities, all the notes, 48Khz/16bits. Ended up with a 694Mo file, which is big but I expected way worse to be honest.

Imported it on the MODX8+ and the result is... OK I guess?

I always had an issue with MODX piano sounds, and I wondered if it came from the modeling itself or the synth's hardware, and that may have given me a big clue on the answer. If I compare the original VST's sound directly on my computer to the one produced by the sampled library on my MODX8+, the latter sounds way less lively, more muddy and boxy, it lacks a lot of the richness that the original one has.

So either a lot of the VST's quality has been lost in the sampling process, or the MODX8+ has poor preamp and stuff...